1. Field of the Invention
The present invention relates to an apparatus for and a method of generating an optical recording pulse, and more particularly, to an apparatus for and a method of generating an optical recording pulse having multi-pulse patterns suitable for a high-density and high-speed optical medium.
2. Description of the Related Art
An optical recording pulse having multi-pulse patterns is designed to overcome the disadvantages of a mark edge recording method that enables recording signals to be recorded on an optical medium with high recording density. A mark is formed on the optical medium in response to the recording signals. If the mark is long enough to have a high thermal density when the recoding signals are recorded on the optical medium using the mark edge recording method, the high thermal density causes a trailing edge of the mark to have a width greater than a leading edge of the mark. The widened trailing edge of the mark results in degradation of the optical recording signals recorded on the optical medium when the optical recording signals are reproduced from the optical medium. As a solution to the problem, the optical recording pulse having multi-pulse patterns is suggested. The optical recording pulse corresponding to the recording signal includes multi-pulse chains or multi-pulse trains (hereinafter referred to multi-pulse).
The optical recording pulse having multi-pulse patterns is generated as shown in FIG. 1B when Non Return to Zero Inverted (NRZI) data of FIG. 1A is inputted. As shown in FIG. 1B, the optical recording pulse having the multi-pulse patterns has three levels, that is, a recording power level, an erase power level, and a bias power level. The optical recording pulse having the multi-pulse patterns includes an erase pulse, a first pulse, a multi-pulse, a last pulse, and a cooling pulse.
FIG. 1B shows a bottom bias power level. However, a basic bias power level corresponding to an erase power level shown in FIG. 1B or an intermediate bias power level which is between the erase power level and a bias power level may be provided for a stable recording operation. If the basic and intermediate power levels in addition to the bottom-level bias power are also considered as bias power levels, the number of the bias power levels would be 3.
The erase pulse having the erase power level is used to erase a previous record or perform pre-heating before a recording signal is recorded . The first pulse has the recording power level and is designed to create the leading edge of a recording mark. With regard to a long mark, the multi-pulse is designed to reduce the unevenness of the domain, which may be caused by the thermal density. If the optical medium is a DVD-RAM or a DVD-R, the multi-pulse is inserted into the optical recording pulse having a length of 5T of the NRZI. If the optical medium is a DVD-RW or a CD-RW, the multi-pulse is inserted into the optical recording pulse having a length of 4T of NRZI. T designates a predetermined clock period for the optical recording signal.
The multi-pulse is inserted between the first pulse and the last pulse when the mark is created. The pulses of the multi-pulse alternate between the bottom bias power level and the recording power level. The number of pulses in the multi-pulse is determined based on the length of the recording mark. For example, in the case of a DVD-RAM or a DVD-R, the number of the pulses of the multi-pulse is obtained from a value corresponding to the length of 4T of the NRZI. In the case of a DVD-RW or a CD-RW, the number of the pulses of the multi-pulse is obtained from a value corresponding to the length of 3T of the NRZI.
The last pulse is used to create the trailing edge of the recording mark and has the recording power level. The cooling pulse has the bias power level, and is used to prevent the recording mark from being formed too long and being distorted. Therefore, in a cooling pulse section, a laser power is turned off in accordance with the cooling pulse. The bias power level attained by the cooling pulse can be the basic bias power level corresponding to the erase power level, the bottom bias power level, or the intermediate bias power level between the erase power level and the bottom level.
However, when the optical medium is initialized, the width (TMP) of the multi-pulse has a fixed value as shown in 11T of FIG. 1B. Therefore, there is a limit in obtaining an ideal domain configuration of the mark formed on the optical medium.
To record the NRZI data shown in FIG. 2A on a phase-transformation optical medium, the optical recording pulse having multi-pulse patterns are inputted to a laser diode (LD) unit. The LD unit determines a power level based on the input pulse and heats a recording film of the optical medium. As shown in FIG. 2B, the recording film of the optical medium is in an erase state around 300° C. and in a liquid state or a melting state if the temperature of the recording film rises above 600° C. Then, the cooling speed of the recording film is adjusted so that a desired mark can be formed. Lowering or turning off the power of the laser beam in response to the cooling pulse may effectively cool the recording film. In FIG. 2B, a horizontal axis indicates time, and a vertical axis indicates the temperature of the recording film of the optical medium.
A domain is created on the recording film of the optical medium by a recording procedure as shown in FIG. 2B. The created domain configuration is shown in FIG. 2D. FIG. 2D shows that the leading edge and the trailing edge of the mark on the domain are uneven and unlike the ideal domain configuration shown in FIG. 2C.
The domain corresponds to the mark or a pit formed on the optical medium. Therefore, unless the first, middle, and last portions of the domain configuration are formed evenly and smoothly, jitter increases, and cross erasure of a gap between adjacent tracks occurs. Moreover, cross talk is generated between adjacent tracks during reproduction of the optical recording pulse. The above problems are more serious for a high-density and high-speed optical medium or a land/groove structure optical medium.